Global warming induced by massive emission of carbon dioxide from fossil fuel combustion and resultant or entailed phenomena such as abnormal climate, massive floods, disappearance of permanently frozen ground, unquestionable melting of glaciers, a rise in sea levels, etc., have come to take place worldwide, frequently and highly repeatedly, and the subsistence environment of the human race is rapidly deteriorating and being threatened. There are therefore strongly demanded to create new energy resources and innovative energy saving technologies for overcoming these serious problems on a global scale as urgently as possible. Wind force power generation, photovoltaic power generation using a solar cell, regenerable new energy resources using biomass, further, an energy saving system using a fuel cell, and the like are expected to be technologies to overcome them, and they are coming to be in widespread use.
However, these new energy systems still have problems in efficiency and cost for putting them to actual use on commercial bases, and it would have to be said that there is no actual technology that can realistically, greatly reducing the carbon dioxide emission. In the recently enforced Kyoto Protocol, it is not easy to attain carbon dioxide reduction numerical targets imposed on contracting countries, and in particular it is thought that the attainment of the reduction numerical target in Japan is also skeptical.
Under these circumstances, hydrogen, as an energy medium for getting human society going and moving forward, has come to attract attention as an ultimate clean medium that is combusted to produce only water without emitting any carbon dioxide. And it is much-publicized that realization of so-called hydrogen society is to come. However, hydrogen has a very low energy density and involves big problems in storage and transportation thereof. Further, it is a dangerous compound that leaks and explodes easily, and handling thereof is difficult. It is very difficult to build necessary infrastructure for storage, transportation and supply thereof, and there exist significant bottlenecks or barriers against the realization of wide commercial-base use thereof.
A so-called fuel cell is presently attracting attention for suppressing carbon dioxide emission. The reason therefor is as follows. In thermal-power generation, a power generator is turned with energy from the mere combustion of a fuel for obtaining power from the fuel. In contrast thereto, when the above fuel is reacted with oxygen in a fuel cell, the reaction is carried out at electrodes and through an external circuit generated charges here can be directly obtained as electric power.
In this case, the energy conversion efficiency from a fuel to electric power in the thermal-power generation is approximately 40%, and in the fuel cell, the above energy conversion efficiency is approximately 40 to 70% on the energy basis and the energy conversion efficiency can be increased as compared with the thermal-power generation. However, it is only hydrogen and methanol that can be generally used in the fuel cell, and for using any other fuel as a fuel in the fuel cell, it is required to convert the fuel to hydrogen once by reforming before use, which leads to a higher cost. Hydrogen gas thus is a fuel that in principle has the above various problems, and methanol is easier to handle than hydrogen and can be directly used as a fuel in the fuel cell. However, the energy conversion efficiency of methanol is not so high and is approximately 30 to 40% at the highest.
On the other hand, a solar cell using a crystalline silicon or amorphous silicon semiconductor has been materialized for converting solar energy to electric power. However, it has a problem that its power generation efficiency is low.
The present inventor has had a new and novel idea that a highly efficient photo-electric generation and photophysicochemical cell, which are capable of converting solar energy or other photo-energy to electric power, which can be used as regenerable energy resources and further which can use compounds that are not conventionally usable as fuels, can be used in society as an energy-saving power generation system that can be an alternative for conventional solar cells and fuel cells.
The above “photophysicochemical cell” is a cell system based on a new and novel concept, while there has been a proposal of one which appears to be similar to the concept of the present invention.
For example, Honda, et al. have reported the following. When a titanium dioxide electrode and a counter platinum electrode are immersed in an electrolyte aqueous solution and when light is applied to the titanium dioxide, water is electrochemically photolyzed to generate hydrogen and oxygen and in this case, photoelectric current occurs (so-called Honda-fujishima effect) (Fujishima et al., “Electrochemical Photolysis of Water at a Semiconductor Electrode”, Nature, 1972, vol. 238, P37-38). In this proposed system, however, hydrogen is generated at a counter cathode, so that the photo-energy is almost all converted to hydrogen. This system is hence hardly usable as a photoelectric cell.
Further, JP 53-131445A discloses a semiconductor photoelectric cell in which sunlight is applied to a semiconductor electrode for decomposition of water. In this electric cell, hydrogen gas is generated, so that it is separated from oxygen which occurs simultaneously, and the separated hydrogen gas is once reserved in a tank of a metal hydride and hydrogen is used as a fuel to actuate a fuel cell.
In Japanese Patent Application No. 2004-379280, further, the present inventor have proposed that a nitrogen-containing compounds can be decomposed by light into nitrogen and hydrogen by a similar method. However, the above proposal intends to produce nitrogen and hydrogen, for example, by photolysis of ammonia, and the photo-energy is all converted to hydrogen energy and hence could not be taken out as an effective electric power.
In JP 59-165379A (to be sometimes referred to as “379 publication” hereinafter), its claim specifies a so-called photo-fuel cell in which an n-type semiconductor electrode and a counter electrode are placed in a solution of an organic substance having a current doubling effect, light is applied to the n-type semiconductor electrode to oxidize the organic substance and oxygen is reduced at the counter electrode to produce electric power. That is, the wording of the above claim appears to have proposed the idea of a photo-fuel cell and the photo-fuel cell appears to be known.
The entire technical content described in the 379 publication has now been studied in detail, and it has been hence found that this publication does not at all describe any means or embodiment which enables one skilled in the art to practice or make what appears to be the “photo-fuel cell” recited in the above claim nor enables to specifically materialize the same. Under the Japanese Patent Law, further, it is requested to describe Example(s) that shows how the invention of the photo-fuel cell recited in the claim is actually embodied. Nevertheless, no Examples described in the above publication provide any embodiments of the idea of the claimed photo-fuel cell. That is, the above Examples describe that a semiconductor anode electrode formed of a Cds single crystal or CdS polycrystal as an anode electrode was inserted in a sodium formate solution (fuel), a platinum black electrode was inserted in sulfuric acid and 500 nm light was applied to the above anode whereby a photoelectric current could be generated. However, these Examples were carried out under very unusual and extreme conditions wherein the solution in which the anode electrode was inserted was strongly basic (a pH of 14 or less) and the solution in which the cathode was inserted was strongly acidic (a pH of 0 or less). This corresponds to a case where a voltage of 0.82 volt or more is applied as an aid between the two electrodes. In principle, the above application of an external voltage to a solar cell, a fuel cell or specifically the above “photo-fuel cell” on purpose is almost the same as inputting of external energy to the above cell and is technically meaningless. In the cell in the above Examples, protons (H+) are generated at the anode electrode and H+s are used up at the cathode electrode during the electric cell reaction, so that the pH difference between the anode cell and the cathode cell gradually decreases while photo-electric current is caused to flow, and as a result, the photo-electric current comes to turn off. In addition thereto, the ion exchange membrane and the salt bridge which constitutes a partition between the anode cell and the cathode cell have a proton exchange function, so that the pH difference between the two cells decreases due to proton exchange process which proceeds while the electric cell is left as it is, and it comes to a stop to operate as a photoelectric cell.
As described above, Examples described in the 379 publication fail to show or demonstrate how the proposed “photo-fuel cell” is actually embodied.
Further, as will be shown in Referential Example 1 to be described later, when a confirmation or verification experiment of the “photo-fuel cell” specified in the claim of the 379 publication was carried out using an anode formed of CdS single crystal, naturally, this fuel cell generated almost no photoelectric current value and had no significant practical use.
The “photo-fuel cell” that is thought to be publicly known by the 379 publication is actually on such a level that it operates only when an auxiliary external energy is applied, and it has not been put into practice as a completed invention having an objective and specific constitution in which it operates and generates intended photoelectric current, without applying any external energy. Further, it also involves a problem that the fuel that can be used is limited to very few compounds “having a current double effect”.
It is an object of the present invention to provide a novel photophysicochemical cell in which any fuel can be oxidatively decomposed with a photoanode without applying any one of the above auxiliary external energy, etc., and a photoelectric current can be generated by reducing oxygen at a counter cathode.
It is also another object of the present invention to provide a photophysicochemical cell capable of generating a photoelectric current by decomposing not only those limited few compounds “having a current double effect” but also a wide range of compounds that naturally include alcohols such as methanol and also include urea, ammonia, agarose, glucose, amino acid and, further, biomass such as cellulose.